1. Field of the Invention
The present invention relates generally to molecular medicine and drug screening assays and more specifically to interactions involved in regulating programmed cell death and methods of identifying drugs that alter such interactions.
2. Background Information
Normal tissues in the body are formed either by cells that have reached a terminally differentiated state and no longer divide or by cells that die after a period of time and are replaced from a pool of dividing cells. For example, nervous tissue is formed early in development and the cells of the nervous system reach a terminally differentiated state soon after birth. In general, when nervous tissue is damaged, the nerve cells are incapable of dividing and, therefore, the loss of function due to the damaged nerve cells is not repaired.
In comparison to the nervous system, the skin is composed of stratified layers of epithelial cells, in which the upper (outer) layer of cells constantly is sloughed off and the lower layer of cells divides so as to replace the lost cells. Thus, the skin is an example of a tissue that is maintained in a steady-state, where the number of cells that are lost is equivalent to the number of new cells produced.
In some tissues such as skin, the steady-state is maintained, in part, due to a process of programmed cell death, in which the cells are genetically "programmed" to die after a certain period of time. A cell experiencing programmed cell death undergoes morphologic changes characteristic of apoptosis, including, for example, fragmentation of its DNA and collapse of its nucleus.
Apoptosis is particularly prominent during the development of an organism, where cells that perform transitory functions are programmed to die after their function no longer is required. In addition, apoptosis can occur in cells that have undergone major genetic alterations, thus providing the organism with a means to rid itself of defective and potentially cancer forming cells. Apoptosis also can be induced due to exposure of an organism to various external stimuli, including, for example, bacterial toxins, ethanol and ultraviolet radiation. Chemotherapeutic agents for treating cancer also are potent inducers of apoptosis.
In tissues such as skin and intestine, which are turned-over continually during the life of an organism, the cells forming these tissues undergo programmed cell death throughout the life of the organism. Normally, this process is tightly regulated and the number of cells produced due to cell division is balanced by the number of cells undergoing programmed cell death. However, the regulation of programmed cell death is a complex process involving numerous pathways and, on occasion, defects occur in the regulation of programmed cell death. Given the critical role of this process in maintaining a steady-state number of cells in a tissue or in maintaining the appropriate cells during development of an organism, defects in programmed cell death often are associated with pathologic conditions.
Various disease states occur due to aberrant regulation of programmed cell death in an organism. For example, defects that result in a decreased level of apoptosis in a tissue as compared to the normal level required to maintain the steady-state of the tissue can result in an increased number of cells in the tissue. Such a mechanism of increasing cell numbers has been identified in various cancers, where the formation of a tumor occurs not because the cancer cells necessarily are dividing more rapidly than their normal counterparts, but because the cells are not dying at their normal rate. The first gene identified as being involved in a cell death pathway, the bcl-2 gene, was identified in cancer cells and was shown to function by decreasing the likelihood that cells expressing the gene would undergo apoptosis.
In comparison to cancer, where the likelihood of a cell undergoing apoptosis is decreased, various pathologies are associated with tissues containing cells undergoing a higher than normal amount of apoptosis. For example, increased levels of apoptosis are observed in various neuropathologies, including Parkinson's disease, Alzheimer's disease, Huntington's disease and the encephalopathy associated with acquired immunodeficiency disease (AIDS). Since nerve cells generally do not divide in adults and, therefore, new cells are not available to replace the dying cells, the nerve cell death occurring in such diseases results in the progressively deteriorating condition of patients suffering from the disease.
Numerous genes involved in programmed cell death pathways have been identified and a role for the products of many of these genes has been described. As a result, the cellular pathways leading to apoptosis are being defined. The delineation of programmed cell death pathways provides targets for the development of therapeutic agents that can be used to manipulate the transfer of an apoptotic signal along the pathway. Such agents, for example, can be directed to a step downstream of a defect in a cell death pathway, thus bypassing the defect and allowing a population of cells having the defect to undergo a normal level of apoptosis. Unfortunately, critical steps in cell death pathways remain to be identified. Thus, a need exists to identify the factors involved in programmed cell death pathways. The present invention satisfies this need and provides additional advantages.